1. Field of the Invention
Embodiments of the invention include methods for the production of porous membranes. In particular aspects the methods are directed to producing polymeric porous membranes having a narrow pore size distribution. The manufacture of the porous membranes of the present invention can be efficient and cost-effective. Porous membranes produced using the materials and methods described herein can be utilized in water purification, healthcare, as well as many other industrial applications.
2. Description of the Related Art
Most of the currently available porous membranes for ultrafiltration are being produced by the so-called phase inversion process (non-solvent induced phase separation). These membranes have a more or less large statistical distribution of pores with different diameters, see S. Nunes, K.-V. Peinemann (ed.): Membrane Technology in the Chemical Industry, Wiley-VCH, Weinheim 2006, pages 23-32. This broad pore size distribution has two disadvantages: (1) Such membranes do not permit precise separation of molecules with similar molecular weights. (2) Membranes with a broad pore size distribution often show a large reduction of flux due to pore blocking (or fouling) of the larger pores. This happens, because most of the liquid passes through the pores with large diameter. According to the law of Hagen-Poiseuille the flux is proportional to the fourth power of the pore radius. Therefore, great effort has been expended and complex methodologies used to produce membranes with a low variance in the distribution of their pore size.
The following methods have been described:
Bacterial envelope method. Isoporous membranes have been produced using bacterial envelopes, so-called S-layers (Sleytr et al.: Isoporous membranes from bacterial cell envelope layers, Journal of Membrane Science 36, 1988). Due to their narrow pore size distribution these membranes have good separation properties. However, these membranes are not commercially feasible because they are difficult to produce on large scale and they are not long-term stable.
Electrolytic oxidation of aluminum. Another method used to produce membranes with low variance in the distribution of their pore size is the electrolytic oxidation of aluminum (R. C. Furneaux et al.: The formation of controlled porosity membranes from anodically oxidized aluminium, Nature 337, 1989, pages 147-149. These membranes are produced on commercial scale and they are offered, for example, under the trade name Anopore™. A significant disadvantage of these membranes is that they are very fragile.
Track-etching. Dense films of polycarbonate or poly(ethylene terephtalate) can be transformed into porous microfiltration membranes with narrow pore size distribution by exposing them to fission fragments from radioactive decay with subsequent etching in alkaline solution (track-etched membranes). The maximum pore density is limited by the fact that membranes become very brittle at very high doses.
Breath figures. Another approach to manufacture of isoporous membranes utilizes so-called breath figures (Srinivasaro et al.: Three dimensionally ordered array of air bubbles in a polymer film, Science 292, 2001, pages 79-83). A moist gas stream is directed in a controlled manner over a solvent-containing polymer film. The pores are created through condensation of water droplets on the surface of the polymer film. This method cannot be used to produce ultrafiltration membranes because it is not possible to obtain pores with a sufficiently small diameter.
Self assembly. A newer method for the production of isoporous membranes is based on the ability of block copolymers to self-assemble into well-ordered structures (T. P. Russel et al.: Nanoporous membranes with ultrahigh selectivity and flux for the filtration of viruses, Advanced Materials 18, 2006, pages 709-712). Block copolymers consist of two or more blocks of different polymers, which are covalently linked. Depending on the compatability of the blocks, the blocks will separate when a film is formed from a solution comprising the block copolymer. However, the length scale of separation is limited because the blocks are connected. This process is therefore called microphase separation. Depending on the nature of the block copolymer different morphological patterns may be formed like spheres or cylinders in a matrix or a lamellar structure. In the method described by Russell et al. an A-B diblock copolymer is dissolved in a solvent together with a certain amount of homopolymer B. Through the controlled evaporation of the solvent, films can form on a solid support like a silicon wafer, which have cylinders arranged regularly and perpendicular to the surface. Homopolymer B is extracted from these films using a selective solvent, so that a nanoporous film is generated. The film can now be detached by water and transferred to a porous carrier. This creates a composite membrane with an isoporous separation layer. This method is very complex due to the multitude of steps. This method does not allow the production of membranes on an industrial scale and at a competitive price.
A simpler method for the production of isoporous membranes also based on the self-assembly ability of block copolymers has been described by Peinemann et al. (Asymmetric superstructure formed in a block copolymer via phase separation, Nature Materials 6, 2007, pages 992-996, see also US Patent publication 20090173694 (the '694 application)). In this work, the above described microphase separation of a block copolymer is combined with the conventional membrane formation process of non-solvent induced phase separation. A concentrated block copolymer solution is cast as a film, which is then after a short evaporation time precipitated in a non-solvent, preferably water. By this method an asymmetric membrane has been obtained consisting of a non-ordered porous support covered by a highly ordered nanoporous separation layer. However, the method has been difficult to reproduce. When the procedure described in the '694 application is applied to commercially available purified block copolymers it does not lead to isoporous membranes.